1. Field of the Invention
The present invention relates to a multiband antenna provided with multiple elements correspond to multiple resonance frequencies, and to a multiband antenna system.
2. Description of the Related Art
A conventional multiband antenna used in a mobile terminal has elements having different lengths from each other. The elements generate mutually different resonance frequencies, respectively. The elements have a multiple branch structure in which the multiple elements are branched off.
FIG. 1 is a view showing a monopole antenna 110. As shown in FIG. 1, the monopole antenna 110 is provided with a ground plane 111, an element 112, and a feeding point 113. The length of the element 112 is almost equal to a quarter of a designed wavelength. The element 112 has a spiral shape, a bent shape, a bent shape or the like so that an impedance of the element 112 can match an impedance of a feed line.
A current distribution of the element 112 is approximate to a sinusoidal distribution having a quarter cycle. The current distribution of the element 112 takes on a maximum value at the feeding point 113, while a voltage distribution of the element 112 takes on a minimum value at the feeding point 113.
FIG. 2 is a view showing a multiband antenna 120 having a multiple branch structure. As shown in FIG. 2, the multiband antenna 120 is provided with a ground plane 121, multiple elements 122 (an element 122A and an element 122B), and a feeding point 123. The element 122A and the element 122B share a common portion 124. The element 122A has a shape in which the element 122A is branched off from the common portion 124. The element 122B has a shape in which the element 122B is branched off from the common portion 124. The multiband antenna 120 can deal with two resonance frequencies by means of the element 122A and the element 122B.
FIG. 3 and FIG. 4 are views showing another multiband antenna 130 having a multiple branch structure. To be more precise, FIG. 3 is a view of the multiband antenna 130, which is viewed from a front surface side, and FIG. 4 is a view of the multiband antenna 130, which is viewed from a back surface side.
As shown in FIG. 3 and FIG. 4, the multiband antenna 130 is provided with a ground plane 131, multiple elements 132 (an element 132A, an element 132B, and an element 132C), a feeding point 133, and a short stub 137. The element 132A, the element 132B, and the element 132C share a common portion 134. The element 132A has a shape in which the element 132A is branched off from the common portion 134. The element 132B has a shape in which the element 132B is branched off from the common portion 134. The element 132C has a shape in which the element 132C is branched off from the common portion 134.
The multiband antenna 130 can deal with three resonance frequencies by means of the element 132A, the element 132B, and the element 132C.
Note that, the short stub 137 connects the common portion 134 and the ground plane 131 via a through hole 138, to achieve matching between the impedance of the element 132A and an impedance of a feed line. Meanwhile, a tip portion of the multiband antenna 130 is bent along a bending line in order to miniaturize the multiband antenna 130.
Here, considering a case where a resonance frequency of the element 122A and a resonance frequency of the element 122B approach each other in the multiband antenna 120 shown in FIG. 2, a length from the feeding point 123 to a tip of the element 122A is almost equal to a length from the feeding point 123 to the element 122B. In this case, a strong coupling occurs between the element 122A and the element 122B. Moreover, a current is distributed to a horizontal portion 125 which is branched off from the common portion 124.
Similarly, considering a case where a resonance frequency of the element 132B and a resonance frequency of the element 132C approach each other in the multiband antenna 130 shown in FIG. 3 and FIG. 4, a length from the feeding point 133 to a tip of the element 132B is almost equal to a length from the feeding point 133 to the element 132C. In this case, a strong coupling occurs between the element 132B and the element 132C. Moreover, a current is distributed to a horizontal portion 135 which is branched off from a branching point 136 of the common portion 134.
For this reason, a radiation pattern of the multiband antenna is altered when the current is distributed to the horizontal portions (the horizontal portion 125 or the horizontal portion 135). Specifically, the elements couples to each other, and the resonance frequencies of the respective elements do not meet designed frequencies.
The increasing in a size of the multiband antenna is conceivable to avoid the coupling between the respective elements. However, such an increase in the size of the multiband antenna is not favorable because the increase in the size of the multiband antenna leads to an increase in a size of the mobile terminal.
Therefore, in a case to provide the multiband antenna with the elements corresponding to multiple resonance frequencies while simultaneously attempting miniaturization of the multiband antenna, it is difficult to avoid the coupling between the respective elements.
Alternatively, it is also conceivable to slightly shift a position of branching off the elements from the common portion shared by the respective elements. However, this arrangement requires strict management of manufacturing errors of the multiband antenna. As a result, there is a risk of a drop in a yield rate of the multiband antenna.